Recently, it has been shown that mesophyll conductance plays an important role in determining the rate of photosynthesis in C3 leaves, and should not be left out of leaf photosynthesis models (Ethier & Livingston 2004). Mesophyll conductance (gm) occurs when a difference exists between the partial pressure of CO2 in the intercellular space (ci) and at the site of carboxylation (cc). This research proposes to measure cc and gm in oak and maple through techniques utilizing stable carbon isotopes and electron transport. This research also seeks to determine the effects of changes in atmospheric CO2 and climate on cc and gm through utilizing open-top chambers already in place at the University of Michigan Biological Station, and by setting up similar chambers near Ione, CA in the lower foothills of the Sierra Nevada Mountains. This research will contribute to the improvement of the multilayer biophysical CANOAK model developed by Dennis Baldocchi. A greater understanding of mesophyll conductance and its response to atmospheric changes will allow for better modeling of photosynthesis in future climatic condit ions. As CO2 in the atmosphere increases, it is important to be able to quantify how much excess CO2 in the atmosphere will be compensated by an increase in the sink from forests. Understanding mesophyll conductance will also deepen our understanding of the carbon cycle, and the interactions with the atmosphere driving this cycle. I will investigate the seasonality of photosynthetic carbon assimilation by investigating a possible contributor to seasonal trends in photosynthesis. In the most used physiological model of photosynthesis, mesophyll conductance, the conductance of CO2 from the intercellular space to the site of carboxylation, is considered infinite (Farquhar et al. 1980). However, recent work has shown that mesophyll conductance can limit photosynthesis. I will investigate in the context of three factors that have been shown to affect mesophyll conductance: leaf thickness (Syvertsen et al. 1995), drought (Flexas et al. 2002), and leaf age (Delfine et al. 1999). Delfine SA, Alvino A, Villani MC, Loreto F. 1999. Plant Physiology 199: 1101-1106. Farquhar G, von Caemmerer S, Berry JA. 1980. Planta 149: 78-90. Flexas J, Bota J, Escalona JM, et al. 2002. Functional Plant Biology. 29: 461-471. Syvertsen JP, Lloyd J, McConchie C, et al. 1995. Plant, Cell, and Environment 18: 149-157.